Abstract
Effects of trace TiB2 addition on the solidification microstructure of Ti–48Al–2Cr–2Nb+(0.18, 0.54, 0.9) wt.% TiB2 alloys were investigated by arc melting and electromagnetic levitation melting in this paper. Results showed that the matrix alloy generated a large-grained (~800μm) fully lamellar microstructure, and the grain size decreased with increasing TiB2 additions. With 0.18wt.% TiB2 addition, the grains remained fully lamellar, at about 700μm in diameter. For more TiB2 addition (0.54wt.%), grain size decreased to an average value of 200μm. The grain refinement could be explained by constitutional undercooling and α phase nucleation on the β laths of secondary TiB2/β borides. With 0.9wt.% TiB2, the alloy melt produced primary TiB2 crystals during solidification to nucleate β grains. The β grains in turn nucleated and were transformed to α grains with the generation of secondary TiB2 flakes. This resulted in equiaxed, refined (~200μm) lamellar grains without element segregation. The elimination of element segregation could be attributed to the fine equiaxed dendrites, corresponding to the result of electromagnetic levitation melting quenching experiments. With equivalent B additions, the grain refinement effect was equal, except for the cavity defects.
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